The present invention relates to gas or oil burning arrangements, and particulary, it relates to a fuel admission assembly hereafter called an FAA having a central swirler for use in a tangentially fired furnace.
In modern heat generating apparatus utilizing oil or gas as a fuel, the load operating range over which an FAA must function efficiently is quite large. In order to maximize combustion efficiency and minimze soot formation, prior art furnaces of the tangential type normally operate at excess air levels of 10% to 20% at full furnace capacity and at even higher excess air levels (plus 50%) at reduced furnace load. With increased cost and reduced availability of high energy fuels such as gas and oil, there exists a need to improve boiler efficiency without increasing the production of noxious emissions. There also exists a need to improve low load boiler efficiency in units which use these expensive fuels as swing load units. One way to improve boiler efficiency is to reduce the level of excess air required to insure efficient combustion of the fuel. Inasmuch as efficient operation of an FAA throughout a wide load range is primarily dependent upon the thoroughness with which combustion air is mixed together with the fuel under all conditions of load, a rotational mode is imparted to the air in the FAA or in the furnace itself to enhance the mixing of the air and the fuel. To reduce the required amount of excess air needed to efficiently consume all of the fuel supplied to the furnace, the mixing generated by swirling the air must be increased in proportion to the desired reduction in excess air. Therefore, in a typical tangential furnace only a minor increase in fuel air mixing (swirl) is required at high load conditions where the tangential action is relatively strong but a significant increase is required at low load conditions where little, if any, tangential interaction exists.
At high loads, where maximum flow is experienced, tangential action within the furnace is dominant and it is sufficient to provide optimum mixing of the fuel and air, therefore the proportion of air directed through the swirler of an FAA may be reduced to a minimum. At low loads, however, there is little air flow within the furnace, and a greater portion of the combustion air must be supplied through the swirler of the FAA to compensate for reduced mixing within the furnace.
Finally, it is important that excessively rapid mixing (swirl) of fuel and air not be achieved because this can result in marked increases in oxides of nitrogen production. Therefore, a careful balance must be struck to insure enough mixing for efficient combustion at low excess air but not such intense mixing as to result in increased levels of oxides of nitrogen.